The invention relates generally to the field of sensor supports, and, in particular, to a new and useful magnetometer stabilization device that facilitates continuous collection of broad-band vector magnetic field data including unique low frequency magnetic measurements in the bandwidth of 1 Hz to 25 Hz, without being affected by motion noise. The invention is particularly useful for mineral resource prospecting using active or passive electromagnetic techniques from air, land or water borne vehicles.
The problem addressed by the invention is the continuous vector measurement of time-varying magnetic fields in the 1 Hz to over 10 kHz frequency range by sensors mounted in a transportable housing towed by an aircraft (often called a bird, a drone or a sonde) or mounted on a land or water vehicle. The magnetic field of interest is created as a result of electromagnetic induction (EMI) by currents at these frequencies flowing through the Earth. They are induced there either by a “Primary” magnetic signal from a transmitter and antenna (controlled-source systems) or by natural time-varying geomagnetic fields produced in or reflected from the earth's ionosphere (passive or natural field systems). These induced fields are extremely weak in comparison to the steady geomagnetic field possessed by the Earth. On a stationary platform, the weak time-varying components are easily distinguished from the strong geomagnetic field, but on a moving platform, rotational motions of a vector sensor in a steady field will create variations in sensor output that may be indistinguishable from the time-varying fields produced by electromagnetic induction. The portion of the sensor output due to rotations is generically called “motion noise” even though strictly it is the rotation and not the linear acceleration of the sensor that causes it.
A variety of devices have been employed in geophysical exploration that exploit EMI to detect zones of enhanced electrical conductivity (“Conductors”) in the Earth and to characterize the time-varying nature of electrical conductivity in the earth (“Polarizability”) through the observed magnetic fields. Such EMI measurements are often diagnostic of mineral and petroleum deposits, lithological and structural variations in ground, aquifers and contamination plumes, and man-made objects such as fences, pipelines, ordnance and treasure. A common attribute of most EMI geophysical systems is a need to measure weak, time varying magnetic fields; and many employ a set of three sensors, each sensitive to the component of the field in a different direction (often orthogonal) to reconstruct the complete time-varying magnetic field vector.
The problem the invention is designed to overcome occurs in all cases where a magnetic sensor is fixed to a housing that moves through a static or low frequency field like that of the Earth's. While linear accelerations are not a problem, rotational acceleration of the sensor creates a time varying signal which is in addition to what would be obtained if the sensor were held in a fixed orientation. While the current invention is intended to be deployed in a housing towed from an aircraft, it could be useful in all situations when vector component magnetic field data are acquired from any mobile vehicle since a static background field is always present. Such vehicles include spacecraft, aircraft, ground and subterranean vehicles, marine and submarine vehicles, or any passive or active drone or platform towed from or attached to such a vehicle. The invention also applies to magnetic field sensors at a fixed location where rotational motion may otherwise be introduced by such effects as vibration, or where the magnetic field is to be measured on a moving part, such as on a piece of machinery.
Many sensor technologies are available for sensitive continuous measurement of magnetic fields. Those most suitable for use in EMI measurement systems are vector sensors that record one spatial component of the magnetic field over a specified range of frequencies, possibly (or not) including long term steady field components like the Earth's geomagnetic field. In what follows, we shall use the term magnetometer or sensor for any of these sensor types, despite the fact that this term is sometimes applied to mean only instruments suitable for measuring the essentially steady geomagnetic field, and that magnetometers for sensing time-varying fields often utilize electromagnetic induction in coils and may therefore sometimes not be referred to as magnetometers.
In the invention contemplated herein, the housing of the sensor system is typically towed behind or under an aircraft along traverse lines. In existing state-of-the art devices towed in this way, the sensors are supported within their housings on passively damped suspensions. The damping and restoring forces in these suspensions have the competing roles of reducing rotational noise while roughly maintaining the orientation of the sensors relative to the housing and the tuning of these suspensions represents a compromise between the two roles. At frequencies much below 25 Hz, such suspensions do not provide sufficient rotational isolation to permit useful EMI measurements. Accordingly, attempts using existing systems to lower operating frequencies much below 25 Hz have resulted in unacceptable noise levels.
As will be explained later in this disclosure, the present invention uses a fairly conventional outer vibration isolation system to reduce the linear and rotational accelerations but adds to this an inner system for rotational isolation with essentially no damping or restoring forces to produce new, advantageous and unexpected advantages over the prior art.
There are a number of methods that have been applied to assist in rotationally-stabilizing instrument platforms of all kinds, including active and passive gyroscopic methods, low-friction, highly-balanced gimbals, and spherical bearings. In airborne geophysics, gyro stabilized platforms have been used in measuring airborne gravity and gravity gradients, and separate patents exist for these technologies. However these systems focus on eliminating linear accelerations rather than rotational accelerations. For EMI measurements, it is the rotational accelerations that are more problematic. The inner isolation system focuses on eliminating these by combining three independent techniques; un-damped single-point suspension, dynamic balancing and inertial gyroscopic stabilization.
The problem of orientation stabilization is shared by the cinema industry where cameras mounted on moving platforms may be subject to unwanted rotations that render the images unusable. The present invention borrows the basic technique of gyro stabilization as developed by that industry. However, stabilization methods used in the cinematic industry are not precise enough and are also problematic for our purpose. Accordingly, the techniques are refined in two main ways. Firstly, the stabilization is made more precise so as to achieve the necessary noise level for useful EMI measurement by the design of the mount and the means by which the assembly it carries is balanced. Secondly, additional magnetic shielding techniques are employed to minimize the effects of electromagnetically-noisy cinematic stabilization equipment on the ultra-sensitive magnetic sensors.
In addition, there are well known linear motion-isolation technologies, including flotation, air bearings, bungee suspension, combinations of springs and shock absorbers and schemes for active signal compensation. Linear motion-isolation can also improve rotational stability by reducing the torques on the platform that result from the application of those linear accelerations to an imperfectly balanced instrumentation platform. Likewise, rotational stability is further enhanced by improvements in platform balance.
The invention contemplated herein is different from existing devices in at least three ways that will become apparent later in this disclosure. First, in its rotational isolation solution, it dispenses with the restoring forces and damping used in existing systems. Instead, the sensors are fastened to a rigid instrument platform that is allowed to float freely on a single spherical air bearing. Thus, the assembly is free to rotate in any direction about a precise centre of rotation and can maintain its orientation relative to the static background geomagnetic field of the earth even while the housing of the platform turns beneath it. This approach recognizes that it is unimportant to maintain the direction of the instrumentation package relative to the housing as long as provision is made that the suspension can be kept within its mechanical range of operation. Independent AHRS (attitude and heading recording) systems are commercially available and are used in this invention to keep track of the orientation of the sensors relative to the housing and relative to the geographic reference frame. Without the damping friction and restoring forces inherent in existing motion isolation systems, the invention is capable of dramatically improving the low frequency noise caused by rotational jitter of the housing.
A second way in which the invention contemplated herein differs from existing devices is that it deals with the problem of dynamic imbalance of the instrumentation platform by incorporating a dynamic balancing system which maintains the centre of mass of the instrumentation platform precisely at its center of rotation. This system is a novel design that uses active members vibrating at fixed frequencies to assess the level of rotational noise induced by the action of each vibrator in each sensor. It uses this information to adjust balance masses in a feedback loop so as to achieve minimal noise.
A third way in which the invention differs from existing devices is that it allows additional resistance to rotation to be incorporated by the addition of several gyro stabilizers (borrowed from the cinema industry) while dealing with the problems of using such electromagnetically noisy devices in close proximity to sensitive EMI sensors.
An industry exists for routine measurement of electromagnetic induction from aircraft. This industry has provided a number of commercial controlled-source systems including the Geotem, Spectrem and Tempest systems which operate from fixed-wing aircraft and Heli-Geotem, VTEM (see U.S. Pat. No. 7,157,914 to Morrison et al.), Aerotem, THEM, Skytem and the Dighem type systems which are towed below helicopters. These all suffer from rotational noise problems to one degree or another, all employ some form of passive sensor rotation control using restoring forces and dampers of one form or another. None of these use gyro stabilization. None of them operate at frequencies below 25 Hz because the motion-induced noise is too high.
Passive (natural field) airborne electromagnetic measurements suffer from the same susceptibility to motion-induced noise as controlled-source systems at lower frequencies. They were first made by Ward with the AFMAG system (Geophysics, Vol. XXIV, No. 4 (October, 1959), pp. 761-789). More recently, passive measurements have been made with the ZTEM system which according to Geotech operates in the 30-6000 Hz frequency band. See “Field Tests of Geotech's Airborne AFMAG EM System,” Lo, et al., AESC Conference, Melbourne, Australia, 2006. VLF-EM is a passive EM system common to many airborne surveys. VLF-EM operates above 10 kHz. In the VLF band, motion noise is not important because the orientation of the aircraft is stable at these frequencies.
U.S. Pat. No. 6,765,383 to Barringer (2002) describes an airborne magnetotelluric survey system operating in the range of 3 to 480 Hz using a towed bird. Few geophysicists believe Barringer's system could ever work. In that system, the magnetic field is measured with a total field magnetometer and 3 orthogonal axis induction coils. A standard commercial angular motion sensor of limited sensitivity is used to record the bird motion and compensate the signal for angular changes in coil orientation. However, the motion of the bird was not separated from the motion of the coils. U.S. Pat. No. 7,002,349 (2006) also to Barringer describes a similar wingtip system.
U.S. Pat. No. 4,629,990 to Zandee (1986) describes the use of low frequency (sub 30 Hz) EM fields for correcting the relative locations of a transmitter and receiver in controlled source airborne systems, but discounts the possibility of using the low frequency data for measuring electromagnetic scattering due to currents induced in the Earth.
Non-electromagnetic airborne geophysical measurements are made from inertially stabilized platforms. Sander Geophysics' Airgrav airborne gravity system measures gravity from a 3-axis Schuler-tuned inertially stabilized platform. U.S. Pat. No. 6,883,372 to van Leeuwen et al. discloses similar technology in a gravity gradiometer of BHP Billiton Innovation Pty Ld. of Melbourne, Australia. Other airborne gravity/gravity gradiometer systems are operated by Bell Geospace, Arkex and New Resolution Geophysics. A gravity gradiometer developed by RTZ is described in U.S. Pat. Nos. 5,804,722 and 5,668,315 to Van Kann et al.
In a different application, U.S. Pat. No. 7,298,869 to Abernathy describes a gyro-stabilized airborne multi-spectral earth imaging system.
U.S. Pat. No. 6,816,788 to Van Steenwyk et al. (2003), describe an inertially stabilized magnetometer measuring apparatus for use in a borehole rotary environment. In this patent, magnetic and gravity component measurements are made in a borehole. The magnetic sensors measure the components of the magnetic field orthogonal to the axis of the hole, and a gyroscope is used to sense inertial angular motion about the borehole axis. The purpose of the gyroscopes is to provide an inertial reference to measure angular rotation data. This inertial reference is used to correct measurements for the rotation of the probe, or to provide a reference to control a rotary drive mechanism which causes the sensors to maintain a stable orientation. A similar patent, U.S. Pat. No. 6,651,496 to Van Steenwyk et al., describes the use of gyroscopes to obtain rotation information for a probe in a borehole so that the rotation information can be used to correct the orientation of the sensors. In both these cases, only the static magnetic field of the earth was measured, and not the magnetic fields caused by time-varying currents flowing in the Earth.
U.S. Pat. No. 6,369,573 to Turner et al., assigned to The Broken Hill Proprietary Company Limited of Australia (BHP-2002), discloses a towed-bird for use in electromagnetic mineral prospecting that uses a method for reducing sensor rotation. The objective of this patent is similar to the current effort, but the approach uses passive isolation methods with a restoring force (springs) and a damper (fluid). This BHP device consists of two nested spherical shells. Liquid is contained between the inner and outer shells and a sphere has openings through which support strings project for locking to an internal point within the sphere. The strings have one end connected to an internal point within the support sphere and another end connected to a spring. The spring includes a damper for damping movement of the spring. Baffles are arranged in the cavity between the inner and outer shells in which the liquid is contained for damping movement of the liquid.
U.S. Pat. No. 5,117,695 to Henderson et al. (1995) uses a related concept for damping and describes a method for a vibration attenuation employing an assembly designed for the protection of single axis instruments such as accelerometers using a damping fluid.
Other recent electromagnetic prospecting patents have been awarded to Dupius et al. (see U.S. Pat. No. 7,375,529) who uses multiple cores to increase the amount of magnetic flux gathered by a magnetometer.
Jackson describes an invention in U.S. Pat. No. 7,397,417 (2008) that is a passive geophysical prospecting apparatus that uses a magneto-resistive sensor in the 65 kHz-12 kHz range.
Klinkert's U.S. Pat. No. 6,244,534 describes an airborne prospecting system which uses a streamlined bird with manipulated attitude-control surfaces to house a transmitter. The pitch of the bird can be controlled by multiple tow ropes, and the receiver can be either in the same bird or in a separate bird. The bird optionally has a motor and separate propeller.
Whitton et al.'s published patent application US 2003/0169045, describes a method for measuring on-time airborne EM measurements by using a rigid transmitter loop and a separate rigid bucking & receiver coil assembly. Their invention employs passive damping.
Many patents exist for self-contained gyroscopic stabilizers. The devices used in the embodiment described here are manufactured by Kenyon Laboratories and work on the principles described in the 1957 U.S. Pat. No. 2,811,042 to Theodore Kenyon.